Batch type industrial heat treat furnaces are generally classified as either standard atmosphere furnaces or vacuum furnaces. When the furnace heat treats a load of loose ferrous workpieces placed in a tray or basket it is referred to as a batch type furnace to distinguish the furnace from the continuous type where individual workpieces are conveyed on an endless belt through the furnace and heat treated as they are passed through various furnace zones.
In a standard atmosphere batch furnace, the workpieces which are loosely placed in a tray or basket, are loaded into a furnace chamber where the atmosphere within the camber is maintained generally at standard atmospheric pressure. More specifically, the atmosphere is maintained at a pressure slightly in excess of standard atmosphere to prevent oxygen from leaking into the furnace chamber as the workpieces are heated in a furnace atmosphere which is either generally inert or carefully controlled in composition to impart certain disassociated chemical components of the heat treating gas into the surface of the case (recarburizing or nitriding).
Most modern day atmosphere furnaces are heated either by electrical heating elements placed at strategic points within the furnace chamber (such as in an arch in the roof) or, more preferably and economically by gas burners firing their products of combustion into imperforate tubes made from high temperature alloys or ceramic materials which prevent any contact of the oxidizing gases with the reactive metal parts under atmosphere inside the furnace chamber. In some instances, electric resistance heaters are enclosed within the tubes. Heat from the hot tubes or electrical elements is then radiated to the workpiece and because of the limited surface areas involved, radiation heat shields are sometimes provided to speed up the heating time. Also, a fan is usually employed to circulate the furnace atmosphere against the tubes or the resistance elements and thence to the workpiece. While fans are obviously an improvement in reducing heating time, the convective heat transfer is obviously limited to the small surface area of the tubes or the resistance heaters and the velocity of the atmosphere gases wiping against the tubes and thence circulated within the workpiece basket. Generally speaking, the heating time is fundamentally limited because of the relatively small surface area of the heat source which limits the convection and radiation heat transfer. Heating by convection is further limited by the low velocity of the circulating atmosphere gases. To effect cooling of the workpieces after heating, a second quench chamber is usually provided in standard atmosphere furnaces. However, where single chamber furnaces are used or the heat treat process does not call for either furnace cooling or liquid quenching, cooling arrangements similar to those used for vacuum furnaces are employed.
Heat treat processes which alter the composition of the case of the workpiece to provide a hard surface which must be controlled to close tolerances (i.e., transmission gears, cams, etc.) are usually carried out in vacuum furnaces. During the heat treat process, the product gas disassociates at the elevated temperature and is drawn into or diffused into the case of the workpiece by a vacuum drawn on the chamber. Because the furnace chamber must be vacuum sealed, it is typically insulated by a water jacket casing. Because of sealing problems encountered with the water jacket, the gas fired radiant tubes used to heat the standard atmosphere furnaces cannot be used in vacuum furnace. Thus, all conventional vacuum furnaces typically use electric resistance heaters to heat the work. This results in a more expensive, at least at this time, process than that which would otherwise occur if gas burners could be utilized to heat the work.
Further, case heat treating processes generally require fast cooling of the workpiece after the case has been properly treated at an elevated temperature. When single chamber vacuum furnaces are utilized or when the heat treat process prohibits quenching, significant alterations of the vacuum furnace chamber are required to achieve the desired cooling rates by atmosphere cooling. Generally such arrangements are characterized as expensive, closed-loop recirculation systems which are situated outside the chamber. Such systems necessarily include a heat exchanger, duct work from the chamber to the heat exchanger and back, seals for the duct at the chamber and rather large fans or blowers to pump the atmosphere from the chamber through the heat exchanger and back again at sufficient velocities to achieve the required cooling rates. In addition, the furnace chamber is modified to include baffles or nozzles or baffles and nozzles to direct the furnace atmosphere at high velocity against the work and inherently, depending upon the geometric configuration of the parts and their stacked relationship within the basket, the nozzles and/or baffles may prove satisfactory for a certain part and unsatisfactory for another part thus requiring adjustment, etc. An example of a vacuum furnace employing such an arrangement as disclosed in U.S. Pat. No. 3,565,410 to Scherff.
Unrelated to industrial heat treat furnaces, but within the broad classification of the furnace art, are annealing coil cover arrangements which have long been used for annealing coiled steel strip produced in the steel mill processes. The box annealing concept bears some physical resemblance to the furnace disclosed in the present invention in that multi-stand annealers use an imperforate cover placed over stacked coiled strip surrounded by a box or stand which heats the cover. A fan is provided in the base of the stand to circulate the atmosphere within the cover and maintain some positive pressure to prevent leakage into the cover. The applications are entirely dissimilar. The annealing cycles are at a significantly lower temperature with much longer processing times than that acceptable for heat treat furnaces. The covers are thick massive steel objects which, along with the stand covers, must be lifted and replaced for each batch of work treated. Sand or loose fibrous seals are employed to seal the open end of the cover, and as noted, the fan is operated to cause a slight positive pressure to leak cover atmosphere past the seal. Such seals are not suitable for heat treat furnaces. They can't hold a vacuum, nor can they withstand the pressures generated in the heat treat furnaces without experiencing a leakage which, while acceptable in the steel mill annealing process because of the surrounding stand, can not be tolerated in heat treat furnaces. Thus, while high momentum heating has been used to speed the heating on the exterior of coil covers, such as that disclosed in one of my prior inventions, for example U.S. Pat. No. 4,142,712 issued Mar. 6, 1979 to Hemsath et al, the products of combustion have not been directed at the sand seal nor have high convective heat transfers occurred within the cover because of the seal integrity problem because such covers have to be movable.
Also bearing some physical resemblance to the present invention and within the heat treat furnace art are muffle furnaces where a thick walled pipe member is structurally anchored at both of its ends to the furnace casing, thus defining a space between the pipe member and furnace casing used to heat the pipe member and the work placed therein. While such furnaces are suitable for certain applications involving continuous furnaces or furnace zones used in continuous furnaces, they are not widely used as simple chamber batch type furnaces because of, among other things, the excessive processing times to heat and core the work vis-a-vis the relatively thick walls of the muffle and the inability to use elastomer seals to efficiently seal the opening.